The following description of the background of the invention is provided simply as an aid in understanding the invention and is not admitted to describe or constitute prior art to the invention.
Reverse triiodothyronine ((2S)-2-amino-3-[4-(4-hydroxy-3,5-diiodophenoxy)-3-iodophenyl]propanoic acid) (rT3) is a non-active isomer of triiodothyronine (T3). T3 and rT3 are both derived from thyoxine (T4) through the action of deiodinase as follows:

Both T3 and rT3 bind to thyroid hormone receptors. When T3 binds, the receptors are stimulated, thus increasing metabolic activity. Upon binding, rT3, unlike T3, does not stimulate thyroid hormone receptors. Thus, rT3 does not stimulate metabolic activity of the target cell, and in fact, blocks receptor sites from T3 activation.
An excess if rT3 may result in widespread shutdown in T3 binding, a condition called reverse T3 dominance. Reverse T3 dominance results in reduced body temperature, which slows the action of many enzymes, leading to a clinical syndrome, Multiple Enzyme Dysfunction, which produces the effects seen in hypothyroidism.
Further, the process of 5′ monodeiodination that converts T4 to T3, and rT3 to diiodothyronine (DIT) is inhibited in a wide variety of conditions, including fasting, malnutrition, poorly controlled diabetes mellitus, trauma, surgery, and systemic illness. Consequently, the serum T3 level typically decreases, and the rT3 level often increases in these circumstances. Thus, the ratio of T3 to rT3 is an important diagnostic marker for the metabolism and function of thyroid hormones and related compounds in clinical chemistry.
Assays for T4, T3, and related compounds (including rT3) have been developed and are used to evaluate thyroid status or to optimize therapeutic dosages. Assay formats include radioimmunoassay and mass spectrometry. For example, Hantson et al. reported quantitating derivatized thyroid hormones via GC-MS (Hansen et al., J. Chromatogr. B (2004), 807:185-192); Zhang et al. reported quantitating T3 and rT3 in human serum via SPE-ESI-MS/MS (Zhang et al., J. Am. Soc. Mass Spectrom. (2005), 16:1781-86); Tai et al. reported quantitating T3 in serum via SPE-HPLC-MS/MS (Tai et al., Anal. Chem. (2004), 76:5092:96); Couldwell et al. report mass spectrometric analysis, including fragmentation spectra, of rT3 in standard organic solvents by ESI-MS/MS (Couldwell et al., Rapid Comm. Mass Spectrom. (2005), 19:2295-2304); Wang and Stapleton report quantitation of rT3 in spiked bovine serum samples via SPE-LC-ESI-MS/MS (Wang and Stapleton, Anal Bioanal Chem (2010), 397:1831-39).